This invention relates generally to book binding, and more particularly it relates to a device for piercing thread staples into folded sheets in which the lengths of thread corresponding to stitches or staples to be formed travel along a straight line together with the folded sheets to be stapled.
According to German publication DWP No. 95 831, a device of this type is known which is stationarily arranged above the feeding track for the sheets and includes three pairs of stitching needles travelling along a circular path on parallel crank gears, whereby the needles assigned to the parallel crank which runs to the lowermost position pierce the sheet fed on a straight line and travel along with the sheet for a relatively small part of its movement.
The disadvantage of this known drive results from the fact that even if the engaged needle moves uniformly with the sheet, the circular path circumscribed by the needle point does not match the rectilinear movement of the sheet. Accordingly, the speed difference during the piercing interval of the needles has to be corrected by additional matching gears. Inasmuch as the working speed of the device is about forty staples per second, the adjustment of the speed produces excessively high acceleration forces which due to the reversal of the mass movement forty times per second can be mastered only by the use of high precision manufacturing processes. Due to this high load, measures have to be found for preventing any wear of the movable component parts since otherwise excessively large perforations would result in the sheets.
Another disadvantage of the known device is connected with the design considerations for the drives of the sheet feeding means on the one hand, and of the parallel crank gears on the other hand. Even if the connecting members of the two drives are limited to a minimum the occurrence of play points between the run of the sheets and the run of the needles, and thus a deterioration in the accuracy of the piercing action, can be permanently avoided only with difficulties and at high costs.
The stitching or piercing step in this known system has to take place only during the half-cycle of the staple forming process. This requirement necessitates an excessively high stitching speed. In general, the speed limit attained by this known drive can hardly be exceeded.
Still other disadvantages arise in the event of an emergency stoppage when large masses have to be braked while the needles are in contact with the sheet and when in order to work without stitches, the gears have to be removable.